Sam and Dean Winchester are often paired with the bride. They see her as a potential threat or an ally in their fight against Lucifer. Their interactions with her are full of tension and mistrust at first, but can develop into more complex relationships. Castiel is another. His angelic nature clashes with the bride's connection to Lucifer, and this creates some interesting dynamics as he tries to understand her and perhaps save her from Lucifer's influence.
Well, a common theme is the exploration of good and evil. The bride might be in a constant battle within herself, as she's tied to Lucifer who represents evil, but she may have her own sense of good. Also, the theme of sacrifice. She could be put in a position where she has to sacrifice her own desires or even her life to prevent Lucifer from causing more chaos. And there's the theme of identity. Figuring out who she is now that she's Lucifer's bride and how that changes her relationships with others.
Another theme is self - discovery. As a human, Lucifer has to figure out who he is without his devil powers. He might discover new aspects of his personality, values, and beliefs. This can lead to some really interesting character development in the fanfiction, as he navigates the human world and tries to understand his place in it.
Well, regarding whether 'Fallen Lucifer' is a popular fiction book, it really depends on the niche it caters to. If it's in the realm of religious or dark fantasy fiction, it could have a following. As for what it's about, if it sticks to the traditional Lucifer narrative, it may tell the story from Lucifer's perspective, perhaps showing his disillusionment with heaven and his journey into becoming a fallen angel. It could also introduce unique twists like new relationships or powers that he discovers after his fall.